This invention relates to a class BC power amplifier which is intended for use in amplifying devices where the efficiency is of primary importance and the use of an output transformer is not obligatory.
A known class BC power amplifier comprises a symmetrical push-pull preamplifier and a pair of intermediate preamplifiers, the outputs of which are connected, respectively, to both power branches of the amplifier, the one power branch being connected to the positive poles of a pair of supply sources, while the other power branch is connected to the negative poles of the supply sources, both power branches and their corresponding intermediate preamplifying stages being totally symmetrical in which corresponding transistors are of opposite conductivities, wherein the outputs of the syymmetrical push-pull preamplifier are respectivly connected to the inputs of the intermediate preamplifiers, the outputs of which are respectively connected to corresponding transistors operaring in class B.
A drawback of this class BC power amplifier lies in that with the common collector connection circuit of th end transistors there cannot be used a control circuit with saturated transistor which can protect the power transistors from breakdown in transient processes caused by the inductive character of the load and it is necessary to build-in a special circuit for this purpose. Another drawback lies in that the end transistors are connected by a common collector circuit and the attacking transistor, which operates in class A conditions, must provide an amplitude greater than the whole output amplitude, which is impossible and leads to an incomplete saturation of the end power transistors, i.e. to a reduction of the maximum possible power and to great distortions of the output signal in the attacking stage itself. A further drawback lies in that the transistor, which operates in class A conditions, must be of a high-voltage type.
There are known circuits of class BC power amplifiers in which the transistor, operating in class C, is controlled by means of a Zener diode by the transistor operating in class B conditions. Thus, the aforementioned drawbacks are avoided, but this leads to a considerable reduction of efficiency, this effect being particularly strong in the range of commutation of the supply sources. The latter is a great drawback for such a control circuit for the transistors, operating in class C conditions, since the amplifier operates the longest time namely in the range of commutation. Thus, the most important characteristics of the class BC amplifier is affected, i.e. its high average statistical efficiency.